It is known from British Patent No 1,195,761 to Wacker-Chemie, that organohalosilicon compounds can be prepared by reacting halosilicon compounds with organosiloxanes which do not contain an Si-bonded halogen in the presence of phosphonitrile chloride. In Example 9 of the British patent, .alpha.-trimethylsiloxy-.omega.-chlorodimethylpolysiloxane is prepared by reacting trimethylchlorosilane and octamethylcyclotetrasiloxane in the presence of phosphonitrile chloride. According to the teachings of the British patent, substantially greater amounts of halosilicon compounds and phosphonitrile chloride were employed than is employed in the present invention. Moreover, the British patent does not describe the preparation of diorganopolysiloxanes having terminal triorganosiloxy units. Furthermore, the British patent does not describe a process for controlling the viscosity and molecular weight distribution of the diorganopolysiloxanes.
Therefore, it is an object of the present invention to provide a process for preparing diorganopolysiloxanes having terminal triorganosiloxy units in which the viscosity of the diorganopolysiloxanes can be controlled. Another object of the present invention is to provide a process for preparing diorganopolysiloxanes having terminal triorganosiloxy groups from readily available organosilicon compounds, at relatively low cost and in good yields. A further object of the present invention is to provide a process for preparing diorganopolysiloxanes containing terminal triorganosiloxy units which have a narrow molecular weight distribution.